1. Field of the Invention
This invention relates to a hydraulic braking system for use in an automotive vehicle, and more particularly to a hydraulic braking system in which the wheels of this vehicle are prevented from locking.
2. Description of the Prior Art
An apparatus with a valve means of the type described above is known as a so called anti-skid apparatus which prevents the wheels from locking in braking operation. A typical apparatus of this type is disclosed, for example, in U.K. Pat. No. 1,542,330. In this apparatus, when the wheels are in the locking condition, the communication between the master cylinder as the hydraulic braking pressure generator and the wheel brake cylinders is cut off by the valve means and the communication between the regulator valve and the wheel brake cylinders as well as the communication between the reservoir and the wheel brake cylinders is controlled by the valve means. As a result, the hydraulic pressure in each of the wheel brake cylinders is increased or decreased to effectively prevent the road wheels from locking. During this time, since the hydraulic pressure from the regulator valve does not act to operate the master cylinder, even if fluctuation of the hydraulic of the hydraulic pressure of the regulator valve is caused by intermittent activation of the valve means, the brake pedal is maintained in its depressed position without causing any unpleasant shock thereon. Furthermore, since the hydraulic pressure of the regulator valve is controlled in dependence upon the hydraulic pressure of the master cylinder so as to become lower than the hydraulic pressure in the power hydraulic source at the voluntary value, the control of the anti-locking operation is apt to do.
In accelerating operation, it is necessary to prevent a slip on driven wheels for providing an optimum accelerating performance. However, because the above-mentioned apparatus is not able to generate the hydraulic pressure from the regulator valve when the brake pedal is not operated, it is not able to prevent the slip on driven wheels during starting or accelerating operation. The regulator valve is not provided therein for such a purpose, but a connecting valve may be disposed in a hydraulic passage connecting the hydraulic braking pressure generator to the wheel brake cylinders provided on the driven wheels, at a position upstream of the above-described valve means disposed in the hydraulic passage, as disclosed in U.S. Pat. No. 4,565,411.
According to the hydraulic brake system described above, the wheels are prevented from locking in braking operation by suitably controlling the hydraulic braking pressure applied to the wheel brake cylinders by means of the valve means, and the slip of the driven wheels is prevented during starting or accelerating operation of the vehicle by means of the valve means and the connecting valve which normally interrupts the flow between the wheel brake cylinders of the driven wheels and a pressure accumulator, i.e. the hydraulic power pressure source. In the above-described hydraulic brake system, a single connecting valve may be disposed in he passage connecting the hydraulic braking pressure generator to the wheel brake cylinders of the driven wheels.
However, in the case where a diagonal dual circuit is employed, in which a first passage connects the hydraulic braking pressure generator to one of the wheel brake cylinders of the driven wheels and one of the wheel brake cylinders of the free wheels, and a second passage connects the hydraulic braking pressure generator to the rest of the wheel brake cylinders, the connecting valve must be disposed in each of the first and second passages, as in the case where all of the wheels are driven. In these cases, therefore, at least two sets of the connecting valve are needed to be disposed and a corresponding arrangement of passages is necessary, so that the braking system as a whole becomes costly, large and heavy.